This project is designed to investigate age-group differences in cognitive performance during adulthood and old age, in order to identify the underlying causes of these differences. Age-group differences in cognitive performance are well-documented, but the underlying causes of these differences have been underinvestigated and are not as well-understood. Cognitive sufficiency, or at least adequacy, is essential for successful adaptation to changing environmental and personal stresses, and is therefore especially important in old age, particularly in that range designated as old-old (85+). The understanding of cognitive differences in adulthood and old age consequently has profound implications for physical and mental health and illness, especially for the elderly. Given the present state of our knowledge about cognitive development, underlying causes can be identified only through theoretical analysis. Therefore, for the proposed research, both the age-groups to be studied and the tasks to be presented were selected on a theoretical basis. The age-groups are young, middle-aged, old, and old-old adults. They will be given 6 tests of thinking and intelligence, 5 tests of problem solving, 2 tests of classification, 7 tests of memory, a test of depression, and a questionnaire assessing 9 demographic variables (gender, age, urban-rural residence, marital status, employment status, present or former occupation, education, health, and use of psychoactive medications). Strong predictions are made about differences among the young, middle-aged, and old age-groups in patterns of performance across tests and on individual tests. The predictions will be assessed with confirmatory factor analyses and multivariate and univariate analyses of variance. Effects of moderator variables will be assessed in these analyses and in orthogonal polynomial regression analyses. Although strong predictions cannot be made about the old-old group, their data will be included in all the analyses, except the confirmatory factor analyses (an exploratory factor analysis will be used for the data of this group because confirmatory factor analysis requires strong predictions). The proposed analyses test the theoretical expectations about the causes of age-group differences and explore ways to modify and extend the underlying theory. Implications for practice and intervention will also be considered.